Recording elements comprising write-once thin film alloy layers

ABSTRACT

Optical recording and record elements are provided. The elements comprise a write-once amorphous thin-film optical recording layer of an alloy having a composition within a polygon in a ternary composition diagram of antimony, zinc, and tin; wherein 
     (a) the composition diagram is according to FIG. 7 herein; and 
     (b) the polygon has the following vertices and corresponding coordinates in atom percent: 
     
         ______________________________________                                    
 
    
              Coordinates                                                      
Vertices   Sb            Sn    Zn                                         
______________________________________                                    
A          98             0     2                                         
B          54            44     2                                         
C          30            44    26                                         
D          30            10    60                                         
E          70             0    30                                         
______________________________________

This is a division of application Ser. No. 058,722, filed June 5, 1987now U.S. Pat. No. 4,774,170.

FIELD OF THE INVENTION

This invention relates to recording elements and recording methods.

BACKGROUND OF THE INVENTION

Thin film optical recording layers using chalcogenide thin-films andamorphous to crystalline phase transitions have been the subject of manyinvestigations since the early 1970's. The initial interests werefocused on "erasable", and therefore reusable, optical recording layerssince the amorphous to crystalline transition is, in principle, areversible process. Such layers are generally prepared by a vacuumprocess. The layer is amorphous when so prepared. A low power,relatively long duration laser pulse is used to heat a local spot on thelayer to below the melting point for a sufficient length of time tocause the spots to crystallize. These crystalline spots can in turn beheated, by a higher power, shorter duration laser, above the meltingpoint of the crystallized spots to randomize the structure of the spots.The layer is designed such that upon the termination of the laser pulsethe cooling rate of the heated spot is high enough that the randomizedstructure is frozen to achieve an amorphous state.

Thus by adjusting the laser power and duration, the state of a selectedarea on the layer can be switched between the amorphous state and thecrystalline state to create a pattern of amorphous and crystalline spotswhich can be used for information storage. Since the phase transition isreversible, the pattern can be erased and replaced with a differentrecorded pattern. Theoretically, this erase-write cycle can be carriedout any number of times.

A principal difficulty is that the rate of crystallization of mostlayers studied is usually too low. For practical applications, it isdesirable to have layers which can be crystallized by laser pulsesshorter than a microsecond (μs). Presently, few materials havedemonstrated such capabilities. For some materials with highcrystallization rates (e.g. Te-Sn alloy), the data retention times areoften not adequate because of the instability of the amorphous state.

Because of the slow crystallization of most materials, thecrystallization step is generally used as the erasure step in erasableoptical recording layers. A laser spot elongated in the direction of thelaser movement is used to give an effectively long duration laserexposure. Such long laser spots cannot be used for high densityrecordings. The amorphizing step, on the other hand, is used as therecording step since this can be achieved with short laser pulse, andhence can be done at high speed.

Very few materials are known for optical recording layers in which theabove described write-erase-write cycle is of practical use. No erasablephase-change type optical recording layers have been commercialized.

A good deal of attention has also focused on so-called "write-once" thinfilm optical recording layers. Write-once simply means that the layerscan be recorded upon only once. Such layers cannot be erased and reusedfor a subsequent recording.

Since thin film optical recording layers are generally amorphous whenprepared, it is desirable to use the crystallization step as therecording step in write-once layers. However, the problem of slowcrystallization prevents the achievement of high data rates. High datarates are critical for write-once layers designed for use withcomputers.

European Patent Application No. 0184452 broadly discloses erasableoptical recording layers of various metal alloys. Information recordingand erasure are said to be achieved by switching the layers between twodifferent crystalline states. The layers are generally prepared in theamorphous states which have to be first converted into one of the twocrystalline states before information can be recorded. Thecrystallization step, achieved by either a bulk heat-treatment or aprolonged laser exposure, is said to have a lower reflectance than theamorphous state. The examples indicate that the materials disclosedtherein have a very low rate of crystallization. This applicationfurther teaches that the optical recording layers disclosed therein areunsuitable for use in the amorphous-to-crystalline transition mechanismbecause of the instability of the amorphous state in general.

Another problem is that many of the chalcogen containing materials whichundergo the amorphous-to-crystalline transition mechanism are usuallycorrosion prone.

The problem is that the prior art has not provided write-once opticalrecording layers which possess the combination of (a) a crystallizationrate less than 1.0 μs, (b) good corrosion resistance, (c) a stableamorphous state and (d) a capability of high rate, high densityrecordings.

SUMMARY OF THE INVENTION

The present invention provides a recording element comprising awrite-once amorphous thin-film optical recording layer of an alloyhaving a composition within a polygon in a ternary composition diagramof antimony, zinc and tin described in FIG. 7 herein; wherein thepolygon has the following vertices and corresponding coordinates in atompercent:

    ______________________________________                                                 Coordinates                                                          Vertices   Sb            Sn    Zn                                             ______________________________________                                        A          98             0     2                                             B          54            44     2                                             C          30            44    26                                             D          30            10    60                                             E          70             0    30                                             ______________________________________                                    

The present invention also provides a record element having

(a) a composition within the above described polygon in FIG. 7; and

(b) a pattern of amorphous and crystalline areas.

The elements of this invention do not suffer the environmental corrosionseen in chalcogen rich thin films. The rate of crystallization of theoptical recording layers is less than 1 μs using practical laser power.The amorphous state is very stable. Thus, recordings on the thin filmare made using the amorphous to crystalline transition mechanism. Thelayers are capable of high density, high rate recordings.

Layers formed from alloy compositions outside of the defined polygon (a)are crystalline as deposited or (b) crystallize too slowly to be ofpractical use.

Especially useful record and recording elements have alloy compositionswithin a polygon in FIG. 7 having the following vertices andcorresponding coordinates:

    ______________________________________                                                 Coordinates                                                          Vertices   Sb           Sn     Zn                                             ______________________________________                                        F          88            4      8                                             G          60           36      4                                             H          34           36     30                                             I          66            4     30                                             ______________________________________                                    

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a description of a schematic recording and readback apparatusfor using the recording elements of the invention; and

FIG. 2 is a schematic cross section of an optical recording element ofthis invention; and

FIGS. 3, 4, 5 and 6 are curves showing some of the experimental resultsof the examples.

FIG. 7 is a ternary composition diagram showing a polygon within whichuseful alloy mixtures in the present invention are found.

DETAILED DESCRIPTION OF THE INVENTION

Recording information on the thin film layers is achieved by focusing aninformation modulated laser beam on the layer thereby forming a patternof crystalline and amorphous areas on the layer.

FIG. 1 shows a schematic of an apparatus for recording information on anoptical recording element 16 of the invention and for playing back therecorded information therefrom. Referring to FIG. 2, recording element16 comprises an overcoat layer 41, amorphous thin film optical recordinglayer 42 on substrate 45. In response to a drive signal, the intensityof a diode recording beam is modulated in accordance with information tobe recorded on thin film 42. The modulated laser beam is collected by alens 14 and collimated by a lens 18 and is directed by means of mirrorelements 20, 23 and 24 to a lens 26 which focuses the modulated laserbeam to a recording spot 28 on the film 42 as shown in FIG. 1.

During recording, the element 16 is spun at a constant rate, e.g. 1800rotations per minute (rpm). As a result, a track of information 30 isrecorded on the optical recording layer in the form of selectedcrystallized areas. As recording continues, the recording spot 28 iscaused (by means not shown) to scan radially inward across the element16, thereby causing information to be recorded along a spiral orconcentric track that extends from an outer radius R_(o) to an innerradius R_(i). The sizes and spacings of the recorded information marksvary in accordance with the information content of the recording laserdrive signal, as well as with radial position on the element 16.

During the readback process, the new information bearing element 16 isspun at the same rate as it was spun during the recording process. Alaser beam 22 from a readout laser is expanded in diameter by means oflenses 34 and 36. The optical path of the readout laser beam is foldedby a beam splitter 21 and mirrors 23 and 24 so that the readout laserbeam is focused to a playback spot on the element 16 by the highnumerical aperture lens 26. The element 16 is assumed to be of thereflective type so that the radiation forming the playback spot isreflected back through the high numerical aperture lens 26 afterinteracting with the information marks recorded on the optical element16. A lens 38 directs reflected laser radiation which has been divertedby the prism beamsplitter onto a detector 40 which produces anelectrical playback signal in response to temporal variations (contrast)in the irradiance of the reflected laser radiation falling on thedetector.

The amorphous thin film optical recording layers of this invention arewritten upon with a coherent beam of electromagnetic radiation ofsufficient energy to convert selected portions of the amorphous film 42to a crystalline state. In the present invention the amorphous thin filmoptical recording layers are of sufficient sensitivity that laser powersof about 2 to 10 mW at laser pulsewidth of 40 to 100 nanosecondsprovides sufficient energy to make the conversion.

Recordings on the amorphous thin film were made with a static pittester.

The static pit tester provides automated facilities in which amicrocomputer controls the sample position, the laser power and thelaser pulse width. Each recording layer is exposed with a 830 nanometerlaser diode in the static pit tester to produce a matrix of spots inwhich the laser power is varied from 4 to 12 mW and the pulse widthvaried from 40 to 30,000 nanoseconds. The suitability of the recordinglayer for optical recording is determined by measuring the change inreflection between the exposed and unexposed areas of the sample, i.e.between the crystalline and amorphous states.

This reflection change is expressed as recording contrast, CT, by thefollowing definition: ##EQU1## wherein R_(c) and R.sub.α are thereflectances of the crystalline and the amorphous states, respectively.A minimum contrast of 5 percent must be achieved for the films to beconsidered useful as optical recording layers.

The thin amorphous film recording layers can be prepared by conventionalthin film deposition techniques such as evaporation, RF (radiofrequency) and DC (direct current) sputtering from an alloy target, andRF and DC co-sputtering from targets of the individual elements.Enhancement of sputtering processes by applying magnetic fields(magnetron sputtering) can also be used. The thickness of the films canbe from a few tens to a few hundreds nanometers depending on compromisesamong factors such as contrast, sensitivity, production rate, materialcost, ease of control, data rate, etc.

Supports which can be used include plastic films, such as polyethyleneterephthalate, polymethyl methacrylate, and polycarbonate, a glassplate, paper and metallic plates.

The practice of the invention is further described in the followingexamples.

EXAMPLE 1

Two amorphous thin film optical recording layers of this invention wereprepared by a sputtering process. A target composed of mixed Sb and Znpowders was pre-sputtered in an 8 mtorr Ar atmosphere for one hour. Thepre-sputtering step was designed to achieve a steady state depositioncondition.

Thin films of about 80 nm in thickness were then prepared by sputteringthe pre-sputtered mix for 3.5 minutes. The sputtered mix was depositedas a thin film on a glass support. The atomic fraction of each componentin the prepared film was determined by inductively coupled plasma atomicemission spectrometry (ICP).

FIG. 3 shows the amorphous to crystalline transition temperature of thinfilms of antimony-zinc comprising (a) 11 atom percent zinc (curve 1) and(b) 8 atom percent zinc (curve 2). The transition temperatures were forfilm (a) 156° C. and for film (b) 117° C. The heating rate was 25milli-Kelvin per second. These high transition temperatures show thatthe amorphous state of the films are stable. This is an importantkeeping property. Spontaneous transition from amorphous to crystallinewould be detrimental to optical recording layers in that the reflectancedifference between the crystalline areas and amorphous areas would belost.

Another sample of the thin antimony-zinc film comprising 8 atom percentof zinc was written upon using the static pit tester described hereinbefore. The writing was in the form of crystallized marks on the films.The film (Sb₉₂ Zn₈) with the crystallized written spots was placed in achamber at 70° C. and 30 percent relative humidity for an acceleratedstability test. After 15 days, the film was examined. We did not observeany phase change or corrosion on the unwritten area or the writtenspots. The film did not have any overcoat as a protective layer againstcorrosion. This test shows that the films of the invention bearingwritten spots are thermally and environmentally stable.

Another film sample comprising 8 atom percent zinc was subjected toperformance tests on the static pit tester. A pulsed semiconductor laserbeam with a wavelength of 830 nm was used for writing. The writingsensitivity and contrast at various powers and pulse widths are shown inFIG. 4. The percent contrast between the initial reflectance of theamorphous state and the final reflectance of the crystallized state isclearly measurable and can thus be read by state of the art laser readsystems. This dat also shows (a) that the thin films can be written uponusing practical laser powers and writing speeds and (b) the reflectivityof the crystalline state is higher than the amorphous state.

EXAMPLE 2

A number of amorphous Sb-Zn thin films with a range of compositions wereprepared according to the method in Example 1. Some representativecompositions are Sb₉₄ Zn₆, Sb₈₆ Zn₁₄ and Sb₈₂ Zn₁₈. The first film canbe written upon with a laser pulse length of 50 ns and power of 6 mW.The second film can be written upon at a laser pulse length of 100 nsand power of 6 mW. The last film can be written upon at a laser pulselength of 1 μs and power of 4 mW.

EXAMPLE 3

Thin films of about 100 nm in thickness were then prepared by sputteringfor 4 minutes as in example 1. FIG. 5 shows the amorphous to crystallinetemperature and reflectance of two different thin films of theinvention.

The amorphous to crystalline transition temperature, the compositionsand curve number are set out below:

    ______________________________________                                        Curve No    Temperature °C.                                                                     Composition                                          ______________________________________                                        1           162          Sb.sub.62 Sn.sub.29 Zn.sub.9                         2           140          Sb.sub.62 Sn.sub.34 Zn.sub.4                         ______________________________________                                    

Comparing to FIG. 3, note that the contrast increases with increasing Sncontent in the films. The reflectance of the crystalline areas wereconsistently greater than the amorphous areas.

Another thin film sample (Sb₆₂ Sn₂₉ Zn₉) was written upon using thestatic pit tester described herein before. The writing was in the formof a pattern of amorphous and crystallized marks on the films. The filmwith the crystallized written spots was placed in a chamber at 70° C.and 30 percent relative humidity for an accelerated stability test.After 14 days, the film was examined. We did not observe any phasechange or corrosion on the unwritten film or the written spots. The filmdid not have any overcoat as a protective layer against corrosion. Thistest shows that the films of the invention bearing written spots areboth thermally and environmentally stable.

Another film sample (Sb₆₂ Sn₂₉ Zn₉) was subjected to performance testson the static pit tester. A pulsed semiconductor laser beam with awavelength of 830 nm was used for writing. The writing sensitivity andcontrast at various powers and pulse widths are shown in FIG. 6. FIG. 6shows the percent contrast between the initial reflectance of theamorphous state and the final reflectance of the crystallized state isclearly measurable and can thus be read by state of the art laser readsystems. This data also shows that the thin films can be written uponusing practical laser powers and writing speeds.

EXAMPLE 4

A number of amorphous thin films with a range of compositions wereprepared according to the method in Example 1. Some of therepresentative compositions were Sb₅₉ Sn₃₆ Zn₅, Sb₇₂ Sn₈ Zn₂₀, Sb₄₅ Sn₂₅Zn₂₀, Sb₄₃ Sn₂₄ Zn₃₃, Sb₆₇ Sn₁₉ Zn₁₄, Sb₅₆ Sn₂₂ Zn₂₂. These films can bewritten upon at a laser pulse length of 50 ns and power of 6 mW.

EXAMPLE 5

Several homogeneous Sb-Zn-Sn alloy sputtering targets with variouscompositions were prepared by hot-pressing. The thin films were preparedby the sputtering process. Some representative compositions are Sb₆₅Sn₂₀ Zn₁₅, Sb₆₀ Sn₂₈ Zn₁₂, Sb₆₀ Sn₂₀ Zn₂₀ and Sb₅₅ Sn₂₀ Zn₂₅. Thesefilms were amorphous and can be crystallized at a laser pulse length of50 ns and power of 4 mW.

None of the thin film optical recording layers in the above examplescould be switched between two different crystalline states.

COMPARATIVE EXAMPLES

Thin film layers were prepared in which the alloy compositions were (a)Sb₄₀ Sn₅₈ Zn₂ and (b) Sb₄₈ Sn₂ Zn₅₀. Film 1 was crystalline whendeposited. Film 2 was amorphous when deposited, but extremely difficultto crystallize. Both of these films are outside the scope of the presentinvention.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

We claim:
 1. A method of recording information, comprising the stepsof:(a) providing a recording element comprising a write-once amorphousthin-film optical recording layer of an alloy having a compositionwithin a polygon in a ternary composition diagram of antimony, zinc, andtin; wherein(i) the composition diagram is ##STR1## and (ii) the polygonhas the following vertices and corresponding coordinates in atompercent:

    ______________________________________                                                   Coordinates                                                        Vertices     Sb    Sn          Zn                                             ______________________________________                                        A            98     0           2                                             B            54    44           2                                             C            30    44          26                                             D            30    10          60                                             E            70     0          30;   and                                      ______________________________________                                    

(b) focusing an information modulated laser beam on the recording layerto form a pattern of crystalline and amorphous areas in the layer. 2.The method of claim 1 wherein the alloy has a composition within thepolygon in ternary composition diagram ##STR2## said composition beingwithin the following vertices and corresponding coordinates:

    ______________________________________                                                 Coordinates                                                          Vertices   Sb           Sn     Zn                                             ______________________________________                                        F          88            4      8                                             G          60           36      4                                             H          34           36     30                                             I          66            4     30                                             ______________________________________                                    


3. The method of claim 2 wherein the alloy comprises Sb₆₅ Sn₂₀ Zn₁₅ ;Sb₆₀ Sn₂₈ Zn₁₂ ; Sb₆₀ Sn₂₀ Zn₂₀ or Sb₅₅ Sn₂₀ Zn₂₅.